1. Field of the Invention
The present invention relates to circuits for automatic gain control (AGC) of signals in a telecommunications receiver, and, in particular, constant compression of signals generated within an AGC circuit.
2. Description of the Related Art
Radio and intermediate-radio frequency (RF and IF) transmitters and receivers commonly employ gain control with feedback to adjust signal levels of each amplifier stage. Analog and digital automatic gain control (AGC) circuits are well-known in the art. Although applications in which RF/IF transmitters or receivers with AGC circuits are used are too numerous to describe in detail, one increasingly popular application is in the field of cellular, wireless, and spread-spectrum wireless communication systems.
The AGC function may be employed to bring a newly acquired signal at the input to the receiver into the dynamic range of the receiver when a communication link is first established. Thereafter, the AGC function continually adjusts the overall receiver gain to compensate for fluctuations in received signal strength associated with fading, interference, periods of xe2x80x9csilencexe2x80x9d between data frames, or similar interruptions of the signal level. The xe2x80x9csettlingxe2x80x9d time of an AGC circuit is the time required by the AGC function to bring the received signal to an optimum level within the dynamic range of the RF receiver. The settling time is typically measured from the time the received signal level first experiences a change in level to the time that the received signal reaches the optimum level.
Such gain control circuitry is primarily employed to maintain a constant level at the output of the series chain of analog components that forms the front end of a receiver, including the amplification and RF/IF demodulation stages. The constant level at the output of the series chain is maintained such that the output (demodulated) baseband signal level of the receiver remains within well-defined limits, even though many factors may vary gain levels within each component of the series chain. For example, in a receiver that delivers audio information to a speaker, the output level of the audio desirably remains constant regardless of 1) variation of the signal level input to the receiver, or 2) variations induced within the receiver components themselves by temperature and/or frequency-dependent influences.
One such influence that occurs is xe2x80x9cgain flatnessxe2x80x9d such that the gain of the circuit component changes as a function of the operating frequency. Most notably, filters often exhibit gain or loss variations across the entire operating frequency range. Another influence is the gain variation across devices manufactured in high volume. Moreover, signal gain (or loss) of these components also varies as ambient temperature changes. The total variation of gain or loss due to frequency variations and/or temperature fluctuations becomes quite large, and possibly out of a specified output level tolerance range, if an AGC function is not implemented within the receiver.
In addition to maintaining a constant signal level output from the series chain of components, the AGC function also adjusts the strength of the signal input to each component along the receiver signal path through the chain. The AGC function adjusts the signal level at points along the path through the chain to an xe2x80x9coptimumxe2x80x9d level that is within the dynamic range of each of the various components. Each component in the receiver chain has an optimum operating range of signal level, which range is bounded on the lower end by its additive noise characteristics and on the upper end by its saturation point. Thus, physical characteristics of realizable components force trade-offs between competing and contrary requirements that are desirably met simultaneously to achieve overall satisfactory receiver performance.
Therefore, a given receiver design may implement the AGC function having three objectives that are desirably met simultaneously. The first objective is to achieve a relatively constant output level for the baseband signal by varying the output signal of one or more components in the series chain as the level of the signal input to the receiver varies. The second objective is to vary the level of the signal as it passes through the series chain so as to 1) be within the optimum operating range of each component, but 2) not be compressed at any one of the components. The third objective is to keep the level of the signal as it passes through the series chain far enough above the noise floor of each component to maintain a specified signal-to-noise ratio (SNR).
Implementation of the AGC function is further complicated by the fact that these 3 objectives must be met in the presence of interference. The total signal power at any point in the signal path and contains varying proportions of the desired signal power and unwanted signal (interference) power. Consequently, relatively simple implementations for an AGC function may mistake an unwanted signal for the wanted, and then either amplify the unwanted signal or attenuate the wanted signal below recognition (i.e., insufficient SNR for a detector of the demodulated baseband signal). Thus, given these three objectives, the AGC function needs to provide an output signal having an acceptable level of overall signal resolution in the presence of signal interference.
A wide variety of AGC systems for RF/IF transceivers are known in the prior art. These AGC systems include both analog and digital designs, and vary considerably according to speed, accuracy, cost, and complexity. The recent growth of standards for wireless and cellular telecommunication systems, and the accompanying growth in the availability of inexpensive microprocessors to process digital signals, have enabled the implementation of digital or digitally controlled AGC systems. Digital AGC implementations have advantages over comparable analog AGC implementations because digital AGC implementations provide powerful and flexible AGC functionality in dynamic signal and interference environments. These benefits are achieved without additional circuitry that may be required by the comparable analog AGC implementations.
Prior art AGC implementations often employ gain control that is distributed throughout the RF and IF demodulation and gain stages of the receiver as individual AGC circuits. An individual AGC circuit commonly employs either a feed-forward or feed-back AGC signal configuration in which each gain stage detects the signal power at either its input (feed-forward signal configuration) or output (feed-back signal configuration). Based on the detected signal power, each individual AGC circuit adjusts input signal level of the gain stage to maintain a constant level at the output of the gain stage. For these individual AGC circuits, each stage includes an autonomously controlled variable gain amplifier that operates independently of other stages. Such configurations may be unstable since compensation of gain in one stage by an amplifier may produce an overcompensation (or other counter-effect) of the signal for subsequent stages.
In addition, traditional AGC circuits with analog variable-gain amplifiers do not allow for a constant compression-point. The compression-point is a measure of an amplifier""s (and, hence, amplifier stage""s) linearity. A comparison of the compression point to a particular signal level at the output of the amplifier is related to the xe2x80x9cheadroomxe2x80x9d (or linearity margin) that the stage exhibits for that particular signal level. To avoid amplifier saturation, it is desirable to reduce the level of the desired signal by reducing the amplifier""s gain while minimizing a corresponding reduction in the compression point of the amplifier stage. However, for most analog-controlled gain stages of the prior art, reducing the desired signal level by a reduction of gain in the amplifier stage generates a corresponding reduction of compression point. Therefore, high input level (i.e., xe2x80x9cstrongxe2x80x9d) signals may require even further gain reduction to maintain signal linearity when the compression point has a corresponding reduction than if the compression point remains constant. While variable gain amplifiers with constant compression point exist within the prior art, they are difficult to implement in, for example, integrated circuits, especially as the operating frequency exceeds 2 gigahertz.
The present invention relates to an AGC function for a multi-stage receiver that provides each stage, if necessary, with a nearly constant compression point. A central controller uses threshold detection to monitor signal power at each stage of the receiver signal path as well as overall signal gain. The central controller adjusts the gain of signals input to each stage in discrete steps, while maintaining overall signal gain for a constant output signal level. The AGC function may be implemented in, for example, an integrated circuit, by switching the gain of each stage""s variable amplifier in discrete steps to adjust signal-to-noise ratio (SNR) and linearity of the signal at each stage to within relatively optimized values. The central controller may determine a gain adjustment for a stage using information from other stages. Monitoring power of the signal at these points as the signal passes through the series chain of components may allow for resolution of the desired signal from unwanted interference signals coincident with the desired signal. Resolution of the desired signal at each stage may further allow for refinement of the gain settings within the receiver.
In an exemplary embodiment of the present invention, an automatic gain control (AGC) function is embodied in, for example, a circuit comprising a series chain of components. The series chain comprises: (1) a first amplifier stage applying a first signal gain to an input signal generate a first gain-controlled signal based on a first AGC signal; (2) a first signal level detector following the first amplifier stage and generating a first signal level value for the first gain-controlled signal; (3) a second amplifier stage following the first signal level detector and applying a second signal gain to the first gain-controlled signal to generate a second gain-controlled signal based on a second AGC signal; and (4) a second signal level detector following the second amplifier stage and generating a second signal level value for the second gain-controlled signal. A controller receives the first and second signal level values and generates the first and second AGC signals. The controller (A) generates an updated first AGC signal based on 1) the second AGC signal and 2) the first and second signal level values; and (B) generates an updated second AGC signal based on 1) the first AGC signal and 2) the first and second signal level values to maintain (i) a level of the second gain-controlled signal within a predefined range and (ii) a nearly constant compression point for the first and second amplifier stages.